This invention relates to a magnetic recording method, and more particularly relates to a method for recording two superimposed signals of different frequencies such as frequency modulated luminance signal and frequency converted chrominance signal in a color video signal recording system, and for optionally recording these two signals together with other signals.
Recently, magnetic video recording techniques have remarkably progressed with respect to increasing recording density. Shortwave and narrow trackwidth recording should be performed in order to increase the recording density. For this purpose, it is required that the influence of self-demagnetization in a shortwave range is reduced, for which it is effective to make the magnetic layer thinner and to increase the coercive force (Hc) thereof. A metal evaporated tape (hereinafter referred to as ME tape), plated tape, ion plated tape, etc., have been proposed as a recording medium having a thin magnetic layer of a large coercive force (Hc).
The ME tape has already partially been put into practical audio recording use. A magnetic layer of the ME tape is deposited onto a base by vacuum evaporation of Co, Fe, Cr, etc. Therefore, it is possible to form a magnetic layer which is 100% metal and having a thickness of between 100 A and 3000 A. The thickness for ordinary practical use is about 1000 A.
On the other hand, the conventional coating-type magnetic tape (hereinafter referred to as a coating tape) has a magnetic layer comprised of a mixture of a magnetic material powder and a binder, and the content ratio of the magnetic material is about 30%. The thickness of the magnetic layer is about 5 .mu.m. Thus, the magnetic layer of the ME tape is much thinner than that of the coating tape.
The ME tape of large Hc suitable for magnetic recording is obtained by performing vacuum evaporation with a device as shown in FIG. 1.
In FIG. 1, numerical designation 1 denotes a tape winder and designation 2 denotes a base film. A base film 3 is fed from the base film feeder 2 and is wound up onto the winder 2 after passing around a roller 4. The roller 4 is controlled in temperature and disposed thereunder is a crucible 5 containing a molten magnetic material such as Co. The vapor of the magnetic material melted at 1000.degree. to 1500.degree. C. flies as shown by arrows 6 from the crucible 5 and a part of it is shielded by a baffle plate 7, so that only a part of the vapor of a controlled projection angle .theta. is deposited on the base film 3. Coercive force Hc of the obtained magnetic tape is controlled by changing the location of the baffle plate 7, i.e. the projection angle .theta. of the vapor against the base film 3 on the roll 4.
The thus obtained ME tape has features such as a very thin magnetic layer, a superior dispersion of magnetic material, a high squareness of its B-H curve which is more than 90%, and a high coercive force.
Such an ME tape is advantageous for recording short wavelength signals for the following reasons:
Losses during electromagnetic transducing which occur relative to the recorded wavelength include a thickness loss and a self-demagnetization loss. Both the losses are relative to t/.lambda. where t is a thickness of the magnetic layer and .lambda. is a recorded wavelength. Therefore, the shorter the recorded wavelength is, or the thicker the magnetic layer is, the larger the losses become. As seen from this characteristic, those losses are especially significant in the short wavelength range. By using a tape having a thin magnetic layer such as the ME tape, those losses are considerably decreased.
Described below are problems to be considered when such an ME tape is used for magnetic recording and reproducing of a wideband signal such as that used in a color video tape recorder (VTR).
Generally in a home-use VTR, a color video signal is recorded in a multiplex signal state composed of a frequency modulated luminance signal and frequency converted chrominance signal whose frequency band is lower than that of the frequency modulated luminance signal. FIGS. 2A and 2B are basic block diagrams for performing the such recording method.
FIG. 2A is a block diagram of a recording system. From an input video signal supplied to a terminal 13 a luminance signal component is separated by a low-pass filter (LPF) 14 and a chrominance signal component is separated by a band-pass filter (BPF) 15. The luminance signal is frequency-modulated by an FM modulator 16. The chrominance signal is converted into a band which is lower than that of the frequency modulated luminance signal by a heterodyne frequency converter 18 with an output from a fixed oscillator (OSC) 17. The frequency modulated luminance signal and the frequency converted chrominance signal are then mixed by a mixer 19 and the mixed signals are amplified by a recording amplifier 20 to be recorded by a video head 21.
FIG. 2B is a block diagram of a reproduction system. A signal transduced by the video head 21 is amplified by a head amplifier 22 and the frequency modulated luminance signal is separated therefrom by a high-pass filter (HPF) 23 and the frequency converted chrominance signal is separated therefrom by a low-pass filter (LPF) 24. The frequency modulated luminance signal is demodulated by an FM demodulator 25 into the luminance signal. The frequency converted chrominance signal is converted into the original chrominance signal by a frequency converter 26 with an output of a variable oscillator 27. The variable oscillator 27 usually comprises an APC circuit or the like, and is operated to remove a phase distortion of the reproduced color burst signal. The thus reproduced luminance and chrominance signals are mixed by a mixer 28, so that the original color video signal is reproduced.
The spectra of the multiplex recording-signal in the NTSC system is shown in FIG. 3, in which the FM luminance signal is represented by 29 and the frequency converted chrominance signal by 30. The carrier frequency of the former is from 3.4 MHz to 4.4 MHz and that of the latter is 630 KHz. The recorded wavelength of these signals ranges from 1 to 60 m. Because of such a wide range of recorded wavelength, the multiplex signal of the frequency modulated luminance signal and the frequency converted chrominance signal (hereinafter referred to as f.sub.Y and f.sub.C, respectively) should be recorded in specified conditions of recording current of each signal.
The method of determining such conditions in the conventional magnetic recording using the coating tape has been as follows:
FIG. 4 shows the characteristics fundamental to the determination of the current of the frequency modulated luminance signal f.sub.Y. Referring to FIG. 4, curves (f.sub.Y), (f.sub.c) and (B) represent the relationships of reproduced f.sub.Y level, reproduced f.sub.c level and cross modulation level expressed by the ratio of reproduced level of the (f.sub.Y -2f.sub.C) cross modulation component to that of f.sub.Y, respectively, with respect to the recording current of f.sub.Y, under the condition of fixing the mixing ratio of recording current of f.sub.C to that of f.sub.Y. The (f.sub.Y -2f.sub.C) component represents one of the components of cross modulation of f.sub.Y and f.sub.C. The reason why this component is taken into consideration is as follows.
Generally, in a VTR, the electromagnetic transducing process of tape to head can be considered to be approximately equivalent to passing through a system having a 3rd order distortion present. When the two-frequency multiplex signal including f.sub.Y =A sin .alpha. and f.sub.C =B sin .beta. is applied to such system, the output component caused by the 3rd order distortion is represented as follows: ##EQU1##
Among the distortion components in the above expression, the 3rd and 6th terms are removed due to the limitation of the transferable frequency band width of the reproduction system. The 1st, 2nd and 4th terms are negligibly small when compared with the fundamental signal components f.sub.Y and f.sub.C. The 5th term which composes the (f.sub.Y .+-.2f.sub.C) component, however, becomes noise of a frequency 2f.sub.C after FM demodulation. This noise should be suppressed so as to be lower than the reproduced f.sub.Y level usually by more than 30 dB.
From the characteristics of FIG. 4, the recording current of f.sub.Y is determined. The optimum condition is obtained when it has the value at a point C in FIG. 4, at which the reproduced level of f.sub.Y becomes maximum.
FIG. 5 shows the characteristics which are used for determining a recording current of f.sub.c. Referring to FIG. 5, curves (f.sub.Y), (f.sub.c) and (B) represent the relationship of the reproduced f.sub.Y level, the reproduced f.sub.c level and the cross modulation ##EQU2## respectively, with respect to the recording current of f.sub.c, under the condition of keeping the f.sub.Y current constant at the point C in FIG. 4. The drop of reproduced f.sub.Y level in the increased recording current range caused by an increased self-demagnetization due to an increase of the f.sub.c current.
In consideration of the decrease the reproduced f.sub.Y level and the increase of the cross modulation, the f.sub.c recording current is determined at a point D in FIG. 5.
The recording currents of f.sub.Y and f.sub.c in a VTR using a coating tape are thus determined and the usual ratio of the recording currents of f.sub.Y and f.sub.c is about 4:1.
On the other hand, in case where there is employed a magnetic recording medium having an extremely thin magnetic layer such as an ME tape, the characteristics of recording and reproducing are quite different from those in the case of conventional coating tape, so that abovementioned method cannot effectively be applied thereto.